This invention relates to a pressure monitoring system and, in particular, a system for monitoring the air pressure within a pneumatic wheel.
A variety of pressure monitors have been proposed. U.S. Pat. No. 5,289,160 discloses a tire pressure monitoring system in which a pressure sensor switch closes if the tire pressure falls below a given valve relative to the external ambient pressure to trigger a warning system. The pressure sensitive switch comprises biasing means the action of which is resisted by the tire pressure but if the tire pressure falls too low relative to atmospheric pressure, the biasing means causes an electrical switch to dose so as to generate a warning signal. This system uses a surface acoustical wave (SAW) device as a transponder to transmit the warning signal to a remote sensor. The possibility of using a SAW device directly on a diaphragm to measure deflection of the diaphragm and thus provide a measure of the pressure acting on the diaphragm, relative to atmospheric pressure, is also mentioned.
U.S. Pat. No. 5,585,571 discloses apparatus for measuring strain by means of a pair of SAW devices. In one embodiment SAW devices are mounted on opposite sides of a diaphragm which is exposed to atmosphere on one side and to tire pressure on the other side. The signals from the two SAW devices are then mixed to provide a temperature compensated signal indicative of the tire pressure.
The present invention aims to improve upon these known systems.
According to a first aspect of the present invention, there is provided a pressure monitoring system comprising a first pressure chamber and a second pressure chamber separated by a diaphragm, a first SAW device having a first resonant frequency mounted on one side of the diaphragm and a second SAW device having a second resonant frequency mounted on the other side of the diaphragm, the first chamber being arranged to be pressurized to a predetermined pressure P1 and to remain at that pressure and the second chamber being arranged to initially be pressurized to a pressure P2 equal to pressure P1 but being subject to change over time, whereby if the pressure P2 in the second chamber falls or rises relative to the pressure P1 in the first chamber, the diaphragm will deflect so one side thereof is subject to tension and the other side thereof is subject to compression, the first and second SAW devices being arranged to sense this deflection by sensing said tension and compression and hence providing an output indicative of the change of the pressure P2 in the second chamber relative to the pressure P1 in the first chamber.
Thus, the SAW devices can be used to provide a measure of the pressure differential between the two chambers, and the pressure in the first chamber can be set to, in effect, store the desired pressure. Thus, if the pressure in the second chamber (which may, for assistance, be in communication with the air pressure within a pneumatic wheel), falls relative to this stored pressure, a warning signal is generated by the pair of SAW devices. Furthermore, the SAW devices provide a temperature compensated signal and can be used as a transponder to transmit the warning signal to a remote sensor.
Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.